DISEASES OF AQUATIC ORGANISMS Vol. 62: 17–26, 2004 Published November 23 Dis Aquat Org Observations on parasitism in deep-sea hydrothermal vent and seep limpets Christina M. Terlizzi, Megan E. Ward*, Cindy L. Van Dover Department of Biology, The College of William & Mary, Williamsburg, Virginia 23185, USA ABSTRACT: Parasite burdens of shallow-water molluscs have been well documented, but little is known about parasite burdens of molluscs from deep-sea chemosynthetic environments (e.g. hydrothermal vents and seeps). Chemosynthetic habitats are characterized by high concentrations of reduced sulfur and, in the case of vents, high heavy metal concentrations. These compounds are nox- ious and even stress-inducing in some environments, but are part of the natural chemical milieu of vents and seeps. To examine parasite types and infection intensities in limpets from vents and seeps we documented parasite burdens in 4 limpet species from 4 hydrothermal vent fields (3 on the East Pacific Rise, 1 on the Mid-Atlantic Ridge) and 1 seep site (Florida Escarpment). Approximately 50% of all limpets examined were infected with 1 or more types of parasites. Limpet parasites were pre- dominantly rickettsia-like inclusions in the digestive and gill epithelia. Limpets collected from the vent field on the Mid-Atlantic Ridge were free of parasites. We detected no histopathological effects that we could attribute to parasites. KEY WORDS: Chemosynthetic ecosystems · Rickettsia · Community ecology · Parasite ecology · Mollusc Resale or republication not permitted without written consent of the publisher INTRODUCTION few studies of microparasites in the deep sea. Informa- tion on parasitism in chemosynthetic ecosystems, such Parasitic infections are common in aquatic communi- as hydrothermal vents and cold seeps, is especially ties, including those involving molluscs (Kim et al. sparse. Hydrothermal vents and cold seeps provide an 1998, Carballal et al. 2001, Poulin 2002). The effects of opportunity to study animals in extreme environments parasitic infection can be devastating to individual (Powell et al. 1999). hosts and to host populations. Infections can impair Deep-sea hydrothermal vents are characterized by growth, reproduction and the ability of the animal to high concentrations of hydrogen sulfide and heavy compete for resources (Calvo-Ugarteburu & McQuaid metals (reviewed in Van Dover 2000). Found along 1998, Ward et al. 2004, this issue). Parasites can affect seafloor spreading centers, hydrothermal vents are host population dynamics by influencing the outcome home to diverse, chemosynthetically based ecosys- of predation and intra- and interspecific competition, tems. Molluscs living at hydrothermal vents typically thereby having an impact on the community structure live at temperatures 1 to 20°C above the ambient 2 to and function of entire ecosystems. Although often 3°C of the surrounding seawater (Van Dover 2000). overlooked by marine ecologists, interactions between Sulfides and heavy metals in vent fluids bathing the parasites and hosts are as important as predation, facil- animal communities are at levels that are normally itation and competition in structuring communities toxic to aerobic organisms, but vent molluscs are (Anderson 1978, Anderson & May 1978). adapted to these conditions (Cosson & Vivier 1997, While parasites and their effects have been well Juniper & Tunnicliffe 1997, Sibuet & Olu 1998). Cold documented for shallow-water communities, there are seeps are a second type of chemosynthetic environ- *Corresponding author. Email: [email protected] © Inter-Research 2004 · www.int-res.com 18 Dis Aquat Org 62: 17–26, 2004 macrofaunal biomass. We studied 3 vent species (Lepetodrilus elevatus from 2 sites on the East Pacific 60°N Rise [EPR; sampled in 1999], L. fucensis from the Juan de Fuca Ridge [JDR; sampled in 1999], and Pseudorim- END ula midatlantica from the Mid-Atlantic Ridge [MAR; sampled in 2001]) and 1 seep species (Paralepetopsis FE 30°N floridensis from the Florida Escarpment; sampled in SP Mid- 2000) (see Fig. 1 for sample locations). These species Atlantic EW Ridge are the dominant limpet species at each site and repre- sent 4 biogeographically distinct regions and 2 types of 0 chemosynthetic habitats. East OA Pacific Rise MATERIALS AND METHODS 30°S All limpets for this study were collected using the Fig. 1. Limpet collection sites. Grey circles: vents; grey star: seep. OA: Oasis, southern East Pacific Rise (17° 25.3’ S, research submersible ‘Alvin’. Limpets were fixed in 113° 12.3’ W, 2582 m; Lepetodrilus elevatus sampled in 1999); 10% buffered formalin for 24 h and stored in 70% EW: East Wall, northern East Pacific Rise (9° 50.9’ N, ethanol. We selected 25 or 30 limpets of each species at 104° 17.5’ W, 2499 m; L. elevatus sampled in 1999). END: random (except Paralepetopsis floridensis,for which ° ° Endeavor, Juan de Fuca Ridge (47 58.00’ N, 129 05.50’ W, only 8 specimens were selected), and numbered, mea- 2220 m; L. fucensis sampled in 1999). SP: Snake Pit, Mid- Atlantic Ridge (23° 22.1’ N, 44° 57.0’ W, 3490 m; Pseudorimula sured (shell length ± 0.1 mm) and weighed (tissue wet midatlantica sampled in 2001). FE: Florida Escarpment weight ± 0.001 g) them. (26° 01.8’ N, 84°54.9 ’W, 3288 m; Paralepetopsis floridensis Limpets were embedded in paraffin wax and sec- sampled in 2000) tioned (5 to 6 µm) longitudinally. Serial sections were stained with Gill’s hematoxylin and eosin (H&E; Stevens 1990) for parasite identification. Infection ment found in the deep sea (see Sibuet & Olu 1998). prevalences (no. of host individuals infected with any Seeps occur in a variety of geological contexts, includ- parasite; Margolis et al. 1982) and intensities (no. of ing brine seeps (e.g. Florida Escarpment seep; Paull individuals of a particular parasite species in each et al. 1984), gas-hydrate seeps (e.g. Blake Ridge; Van infected host; Margolis et al. 1982) were determined Dover et al. 2003), and petroleum seeps (Gulf of quantitatively for each individual and the sex of each Mexico; MacDonald et al. 1990). limpet was recorded. Individuals not infected with any Recently, 2 studies have focused on the parasite parasites were excluded from the calculation of within- burdens of bathymodiolin mussels from chemosyn- site mean infection intensities. Micrographs of para- thetic systems (Powell et al. 1999, Ward et al. 2004). sites were taken using a Spot camera (Diagnostic Both studies documented parasites similar in mor- Instruments); contrasts were adjusted using Adobe phology to those found in coastal molluscs. These Photoshop (Adobe Systems). types of parasites included rickettsia and chlamydia- Pearson’s correlations and the non-parametric like cellular inclusions and extracellular gill ciliates Kruskal-Wallis test where appropriate were under- (Powell et al. 1999, Ward et al. 2004). Prevalence of taken using MINITAB software (Version 13.20, 2000). some types of parasites, such as rickettsia- and The multi-dimensional scaling (MDS) technique chlamydia-like inclusions, was higher in deep-sea (PRIMER v5; Clarke & Gorley 2001) was used to exam- mussels than is typically observed in mussels from ine differences between individuals based on parasite intertidal regions. Seep mussels were more heavily infection, using Bray-Curtis similarities calculated parasitized than vent mussels (Ward et al. 2004), pos- from the non-transformed infection intensities of para- sibly due to the greater stability of the seep environ- sites from each limpet, excluding individuals that were ment compared to the hydrothermal vent environ- not infected with any parasites. The relative positions ment (Sibuet & Olu 1998). of individual points on MDS plots (unitless, 2D spaces) To extend our appreciation of parasite burdens in represent relative similarities of multivariate data for invertebrates in chemosynthetic environments, we each limpet individual, derived from a parasite-abun- chose to examine parasite burdens in deep-sea hydro- dance matrix. Analysis of similarity (ANOSIM subrou- thermal vent and seep limpets. Limpets, although tine of PRIMER v5) was used to determine significant small (typically <1 cm length), are common and abun- differences between groups distinguished by MDS. dant at vents and seeps, and often dominate the ANOSIM provides R statistics: where R > 0.75, groups Terlizzi et al.: Parasitism in deep-sea chemosynthetic limpets 19 are well-separated; where 0.75 > R > 0.5, groups are We identified 3 rickettsia-like gut inclusions infect- overlapping but clearly different; where R < 0.25, ing the cytoplasm of the host cell. The first, referred groups are not separable (Clarke & Gorley 2001). Fac- to hereafter as Digestive Rickettsia I (Fig. 2a), was tors contributing to these differences were determined found in the digestive epithelial cells of the stomach from similarity percentages (SIMPER subroutine in of Lepetodrilus elevatus from EPR vents. These PRIMER v5). basophilic inclusions were generally spherical in The limpet Paralepetopsis floridensis lacks gills. This shape with an average diameter of 15 µm (n = 10). A character, plus the reduced sampling effort for P. flori- membrane separated the inclusion from the host cell densis, forced us to make descriptive rather than quan- (Fig. 2b). Obvious tissue pathology was uncommon, titative comparisons of parasite burdens between seep although in some cases the inclusion could be seen and vent limpets. breaking through the host cell membrane. Another rickettsia-like gut inclusion, referred